The present invention relates to scanning tunneling microscopy, and more particularly to a two-dimensional walker assembly for coarse positioning of a sample to be scanned with respect to an imaging probe of a scanning tunneling microscope (STM).
In 1986 Binnig and Rohrer of IBM Corporation in Zurich, Switzerland successfully demonstrated scanning tunneling microscopy. Their work is reported in the IBM Journal of Research and Development, Vol. 30 No. 4, July 1986, pages 355-369. In this technique an imaging probe is brought to the surface of a sample under study within 4-8 Angstroms, and a tunneling current is generated between the imaging probe and sample under a relatively low bias voltage. Due to the extremely small spacing control required in designing an STM, management of thermal drift and vibration is of great importance. This task has proven to be very difficult. As a result most of the STM designs support a rather limited lateral travel range for the sample surface with respect to the probe tip. Such an STM is suitable for performing one-dimensional analyses where samples under study have a large homogeneous area. However this restriction makes the STM unsuitable for real device applications because in device analyses the probe tip always needs to travel several millimeters horizontally before being properly positioned.
A common approach to accommodate a large X-Y motion is to make use of miniature X-Y micropositioners. However for a one-inch long linear dimension of the micropositioner, the tip may extend about three thousand Angstroms for a one-degree Centigrade change of temperature due to thermal expansion. Since the spacing needed between the probe tip and the sample surface is on the order of 4-8 Angstroms, this expansion due to temperature variation is unacceptable. As a result in order to make a scan of the sample at different temperatures a long delay is required to assure temperature stability at each measurement temperature. This problem has prevented the two-dimensional STM systems from attaining large X-Y excursions that are desirable for device applications.
As shown in FIG. 1 a current motor design 10 for an STM has a base plate 11, typically of a beryllium-copper composition, upon which are mounted two concentric tubes 12, 13 of piezoelectric material, such as PZT (Pb{Ti,Zr}O.sub.3). A probe tip 14 is mounted at the end of the inner PZT tube 12 along the central axis. Via holes are provided through the base plate to allow electrical connections to be made between the PZT tubes and external electrical circuits. The inner PZT tube has an electrode on the outer surface that is segmented into quadrants. The tubes also each have a solid electrode on the interior surfaces. Surrounding the tubes is a cylindrical shield 15 of a material such as brass. Extending from the end of the outer PZT tube are a pair of rails 16, typically quartz rods, upon which a sample holder 17 rests. On the face of the surface holder facing the probe tip is mounted the sample 18 under study. Extending from the end of the shield is a cylindrical viewing tube 19 having an appropriate conductive coating, such as indium-tin-oxide (ITO). An end plate 20 is secured to the base plate by suitable means, such as screws, to hold the entire motor assembly together.
In operation the sample under study is mounted on the face of the sample holder, which in turn is slid onto the rails until it contacts the end of the outer PZT tube that provides Z-translational motion. Current applied to the inner electrodes of the tubes causes the tubes to oppositely expand and contract along the cylindrical axis. Initially the current allows the sample holder to contact the outer tube without contacting the probe tip mounted on the inner tube. Then the currents are adjusted to adjust the Z distance between the probe tip and the sample surface to the desired tunneling distance. Differential currents are applied to opposing outer electrodes of the inner tube to produce appropriate X-Y directional movement of the probe tip with respect to the sample surface to scan the sample. The probe tip motion is limited to about +/-8 micrometers. To scan another portion of the sample surface requires the use of the micropositioners mentioned above for relatively large lateral displacements.
What is desired is a two-dimensional walker assembly for positioning a sample for study over large lateral distances that compensates for temperature drift.